During the construction of wells, cement is used to secure and support casing inside the well and prevent fluid communication between the various underground fluid-containing layers or the production of unwanted fluids into the well.
Various approaches have been developed to prevent failure of the cement sheath. One approach is to design the cement sheath to take into account physical stresses that might be encountered during its lifetime. Such an approach is described for example in U.S. Pat. No. 6,296,057. Another approach is to include, in the cement composition, materials that improve the physical properties of the set cement. U.S. Pat. No. 6,458,198 describes the addition of amorphous metal fibers to the cement slurry to improve its strength and resistance to impact damage. EP 1129047 and WO 00/37387 describe the addition of flexible materials (rubber or polymers) to the cement to confer a degree of flexibility on the cement sheath.
Nevertheless, the above-described approaches do not allow restoration of the zonal isolation once the cement sheath has actually failed due to the formation of cracks or microannuli.
A number of self-healing concretes are known for use in the construction industry. These are described for example in U.S. Pat. Nos. 5,575,841, 5,660,624, 5,989,334, 6,261,360 and 6,527,849, and in the document entitled “Three designs for the internal release of sealants, adhesives, and waterproofing chemicals into concrete to reduce permeability”, Dry, C. M., Cement and Concrete Research 30 (2000) 1969-1977.
Nevertheless, none of these self-healing concretes are immediately applicable to well cementing operations because of the need for the material to be pumpable during placement.
“Self-healing” cements were eventually developed for oil and gas industry applications such as described in US 2007/0204765 A1, WO 2004/101951 and WO 2004/101952 A1. These formulations generally contain additives that react and/or swell upon contact with downhole fluids. When cement-sheath deterioration occurs, exposing the cement matrix or cement-sheath surfaces to downhole fluids, the additives respond and seal cracks or fissures, thereby restoring cement-matrix integrity and zonal isolation. Well cements are potentially exposed to several fluid types during service, including liquid and gaseous hydrocarbons, water, brines and/or carbon dioxide. Thus, depending on the anticipated wellbore environment, it would be desirable to incorporate additives that are able to respond to one or more types of downhole fluids.
Despite the many valuable contributions from the art, it would be desirable to have access to a self-healing set cement that responds to formation fluids that contain high concentrations of gaseous hydrocarbons.